1. Complexometric Titrations

2. Introduction
The simple salt consists of two radicals, e. g. (acidic and basic radicals), for example:
FeCl3, CoSO4, Cu(NO3)2, …etc.
Fe3+, Co2+, Cu2+, ... Basic radical
Cl-, SO42-, NO3-, ... Acidic radical
The complex salt consists of more than two radicals, such as:
[Fe(H3N)6]Cl3, [Co(NH3)4]SO4, [Cu(NH3)4] …etc.

3. The chemical species [Fe(HN3)6]Cl3, [Co(NH3)4]SO4, [Cu(NH3)4], [Co(NH3)4Cl2]Cl, [Co(NH3)5Cl]Cl2 are called metal complexes or coordination compounds.
The formation of a coordination compound from a metal ion is called complexation.
The part [Co(NH3)6]3+ in [Co(NH3)6]Cl3 molecule is called complex species or complex ion.
The formula of the complex species is written within bracket [ ].
The six NH3 groups are called the ligands.
The ligand can be defined as any molecule or ion that has at least one electron pair (Lewis base) which can be donated to a metal ion (Lewis acid).

4. Transition elements exhibit two types of valencies, namely primary valence and secondary valence.
The primary valence is also known as ionisable valence and secondary valence is otherwise known as nonionisable valence.
Anions can satisfy primary valence whereas anions or neutral molecules can satisfy secondary valence.
In modern terms, the primary valence corresponds to the oxidation number and the secondary valence corresponds to the coordination number.
Example: [Co(NH3)3Cl3]

5. Types of ligands Monodentate ligands
When the ligand has only one donor atom, it is said to be monodentate.
It may be a neutral molecule such as
or a negatively charged ion such as

6. Chelation
When the ligand has two or more donor atoms, the terms bidentate, tridentate, tetradentate …etc., generally multidentate frequently used.
The cyclic compound form when the multidentate ligand attached to the central metal atom or ion by more than one coordinating atom, is called chelate.
The process is known as a chelation.

7. Bidentate Ligands * * * * * * Donor Atoms Ethylenediamine (en)
oxalate ion * *
Donor Atoms
ortho-phenanthroline

8. tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+

9. Polydentate Ligands * * * * * *
EDTA
Donor Atoms

11. Polydentate ligand and chelation
EOS

12. The Chelate [Pt(en)2]
Ethylenediamine has two nitrogen atoms that can donate electrons to a single metal ion.
The square planar complex of Pt2+ and ethylenediamine has two ligands with a total of four points of attachment.

13. Complex species are of different types; cationic, neutral and anionic:
[Pt(NH3)4] Cationic
[Pt(NH3)4Cl2] Neutral
[PtCl4] Anionic
The molecules (or ions) of complexes have characteristic stereo shapes such as tetrahedral, square planar, octahedral, …etc., depending on the number of ligands bonded to the central metal.

14. Examples of complexes
Cationic complex ions

15. Neutral complex
Anionic complex ions

16. Significant Definitions
Complex ion: Species where transition metal ion is surrounded by a certain number of ligands.
Coordination number: The total number of monodentate ligands attached to the central metal ion in the complex molecule.
For example, in the complex ion [Cu(NH3)4]2+, as four monodentate ligands are attached to the central metal ion, the coordination number is four.
The coordination number can be also known as the number of sigma bonds between the ligands and the central metal ion.
Coordination numbers from two to nine are known in complexes.

17. Complex ion
Complex ion is formed by the union of a simple ion with either other ions of opposite charge or with neutral molecules as shown by the following examples.
AgCN + CN− [(AgCN)2]− (complex ion)
AgCl + 2NH [Ag(NH3)2]Cl (complex molecule)
CuCl2 + 4NH [Cu(NH3)4]Cl2 (complex molecule)
Complexation process: is a reaction with a metal ion involves the replacement of one or more of the coordinated solvent molecules by other Nucleophilic groups. The groups bound to the central ion are called ligands and in aqueous solution the reaction can be represented by the equation: Mm+(H2O)n + L [Mm+(H2O)(n-1)L] + H2O

18. Stability of complexes
The thermodynamic stability of a species is a measure of the extent to which this species will be formed from other species under certain conditions, provided that the system is allowed to reach equilibrium.
Consider a metal ion M in solution together with a monodentate ligand L, then the system may be described by the following stepwise equilibria:
M + L ML; K1 = [ML]/[M][L]
ML + L ML2; K2 = [ML2]/[ML][L]
ML(n−l) + L MLn; Kn = [MLn]/[ML(n−l)] [L]
The equilibrium constants K1, K2, ..., Kn are referred to as stepwise stability constants or the overall stepwise formation constant K;
K = K1 × K2 ×..., Kn

19. Knowledge of stability constant values is of considerable importance in analytical chemistry, since they provide information about the concentrations of the various complexes formed by a metal in specified equilibrium mixtures; this is valuable in the study of complexometry, and of various analytical separation procedures such as solvent extraction, ion exchange, and chromatography.

20. Complexones Ethylenediaminetetra-acetic acid (EDTA)
The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. I II
The formula of EDTA (I) and the zwitterions (II).

21. The values of pK are respectively pK1 = 2. 0, pK2 = 2. 7, pK3 = 6
The values of pK are respectively pK1 = 2.0, pK2 = 2.7, pK3 = 6.2, and pK4 = 10.3 at 20 ºC; these values suggest that it behaves as a dicarboxylic acid with two strongly acidic groups and that there are two ammonium protons of which the first ionizes in the pH region of about 6.3 and the second at a pH of about 11.5.
To simplify the following discussion EDTA is assigned the formula H4Y: the disodium salt is therefore Na2H2Y and affords the complex-forming ion H2Y2− in aqueous solution; it reacts with all metals in a 1:1 ratio.
The reactions with cations, e.g. M2+, may be written as:
M2+ + H2Y2− MY2− + 2H+

22. Stability Constant of Metal EDTA Complexes
The stability of a complex is characterized by the stability constant (or formation constant) K:
Mn+ + Y4− (MY)(n−4)+
K = [(MY)(n−4)+]/[Mn+][ Y4−]

23. Metal ion Indicators
The success of an EDTA titration depends upon the precise determination of the end point. The requisites of a metal ion indicator for use in the visual detection of end points include:
(a) The color reaction must be such that before the end point, when nearly all the metal ion is complexed with EDTA, the solution is strongly colored.
(b) The color reaction should be specific or at least selective.
(c) The metal-indicator complex must possess appreciable stability, but the metal-indicator complex must, however, be less stable than the metal-EDTA complex.
(d) The color contrast between the free indicator and the metal-indicator complex should be such as to be readily observed.

24. (e) The indicator must be very sensitive to metal ions.
(f) The above requirements must be fulfilled within the pH range at which the titration is performed.
The indicators which form complexes with specific metal cations can serve as 1:1 complexes.
The molar ratio metal: indicator = 1:1 are common, but 1:2-complexes and 2: l-complexes also occur.
The metal ion indicators, like EDTA itself, are chelating agents.

25. Theory of the visual use of metal ion indicators.
Discussion will be confined to the more common 1: l-complexes.
The use of a metal ion indicator in an EDTA titration may be written as: M-In + EDTA → M-EDTA + In
This reaction will proceed if the metal-indicator complex (M-In) is less stable than the metal-EDTA complex (M-EDTA).
The former dissociates to a limited extent, and during the titration the free metal ions are progressively complexed by the EDTA until ultimately the metal is displaced from the complex M-In to leave the free indicator (In).
The indicator color change is affected by the hydrogen ion concentration of the solution.

26. Thus Eriochrome black T, who may be written as H2In−, exhibits the following acid-base behavior:
In the pH range 7 − 11, in which the dye itself exhibits a blue color, many metal ions form red complexes; these colors are extremely sensitive, as is shown, for example, by the fact that 10−6 − 10−7 molar solutions of magnesium ion give a distinct red color with the indicator.

27. Eriochrome black T

28. Murexide

29. Types of EDTA Titrations
A. Direct titration.
The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to pH = 10 with NH4Cl/NH4OH, i.e. the ammoniacal buffer) and titrated directly with the standard EDTA solution.
Mn+ + In [M-In]
Color 1
Color 2
[M-In] + EDTA [M-EDTA] + In
Colorless
Color 1
Mn+ + In + 10 ml of
buffer solution pH = 10

30. B. Back-titration.
Many metals cannot, for various reasons, be titrated directly; thus they may precipitate from the solution in the pH range necessary for the titration, or they may form inert complexes, or a suitable metal indicator is not available.
In such cases an excess of standard EDTA solution is added, the resulting solution is buffered to the desired pH, and the excess of the EDTA is back-titrated with a standard metal ion solution.
A solution of zinc chloride or sulphate or of magnesium chloride or sulphate is often used for this purpose.
The end point is detected with the aid of the metal indicator which responds to the zinc or magnesium ions introduced in the back-titration.

31. Mn+ + EDTA(excess) → [M-EDTA] EDTA + Zn2+ or Mg2+ → [Zn or Mg – EDTA]
EDTA(excess) + Mn+ + In +10 ml of buffer solution pH = 10
Mn+ + EDTA(excess) → [M-EDTA]
EDTA + Zn2+ or Mg2+ → [Zn or Mg – EDTA]
Zn2+ or Mg2+ + In → [M-In]

32. C. Replacement or substitution titration.
Substitution titrations may be used for metal ions that do not react (or react unsatisfactorily) with a metal indicator, or for metal ions which form EDTA complexes that are more stable than those of other metals such as magnesium and calcium.
The metal cation Mn+ to be determined may be treated with the magnesium complex of EDTA, when the following reaction occurs:
Mn+ + MgY2− (MY)(n−4)+ + Mg2+
The amount of magnesium ion set free is equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator.
In the direct titration of calcium ions, Solochrome black gives a poor end point; if magnesium is present, it is displaced from its EDTA complex by calcium and an improved end point results.

33. D. Alkalimetric titration.
When a solution of disodium EDTA, Na2H2Y, is added to a solution containing metallic ions, complexes are formed with the liberation of two equivalents of hydrogen ion:
Mn+ + H2Y2− (MY)(n−4)+ + 2H+
The hydrogen ions thus set free can be titrated with a standard solution of sodium hydroxide using an acid-base indicator or a potentiometric end point; alternatively.

34. E. Miscellaneous methods.
Exchange reactions between the tetracyano-nickelate(II) ion [Ni(CN)4]2− and the element to be determined, whereby nickel ions are set free, have a limited application.
Thus silver and gold, which themselves cannot be titrated complexmetrically, can be determined in this way.
[Ni(CN)4]2−+ 2Ag [Ag(CN)2]− + Ni2+
the equivalent amount of nickel thereby set free is determined by rapid titration with EDTA using an appropriate indicator (murexide, bromo pyrogallol red).